In many industrial processes, the presence and amount of even minute moisture concentrations in flowing gas streams must be detected and measured with a high degree of speed and accuracy. The process of manufacturing semiconductors uses flowing gas streams, for example, and trace moisture concentrations present in those streams affect production yield. If moisture concentrations exceed specified limits, the contaminated gas stream may produce, at considerable expense, an unacceptable semiconductor lot. Thus, detection and measurement of moisture concentrations in industrial processes such as semiconductor production is required because moisture is often critical to the quality of the product made.
To meet the industrial demand, sensitive hygrometers are available which have extremely low detection limits and fast response times. The most sensitive and commercially available hygrometers can detect and measure moisture concentrations on the order of ten parts per billion by volume--although modern, high-purity hygrometers may reach limits of a few ppb. The prior art is replete with a variety of water detection and measurement devices, or hygrometers. These include infrared absorption-type hygrometers, conductivity cells, piezoelectric hygrometers, impedence-type type hygrometers, mirror dew point apparatuses, gas chromatographs (which may include an electron capture detector), and electrolytic hygrometers. A general discussion of such devices is found in U.S. Pat. No. 4,535,620, issued to R. Cunningham.
K. Sugiyama & T. Ohmi, "Ultraclean Gas Delivery Systems--Part I," in Microcontamination at 49-54 (November 1988), discloses that gases with moisture levels on the order of two parts per billion can be produced and that such levels can be measured by Atmospheric Pressure Ionization Mass Spectrometry (APIMS). See also T. Kimura, J. Mettes & M. Schack, "Sub-ppb Analysis of Nitrogen Gas by APIMS," presented at the Technical Symposium of SEMICON EAST 89 in Boston, Mass. (September 1989) (disclosing an experimental setup and a procedure for the analysis of high-purity nitrogen).
The present invention focuses on the electrolytic hygrometer. This type of hygrometer operates under the principles of Faraday's Law and incorporates an electrolytic cell as the analytical component. One configuration of an electrolytic cell (a type I cell) consists of a hollow glass tube with two electrodes helically wound around the inside and covered with an hygroscopic film such as phosphorous pentoxide (P.sub.2 O.sub.5). The two electrodes, one a positive anode and the other a negative cathode, form a double helix. The gas to be measured flows through the cell with a known flow rate.
The water concentration of the gas is determined in the following manner. The hygroscopic film absorbs the water from the gas. A voltage is supplied across the electrodes, which electrolyzes the water in the film into hydrogen and oxygen. The current generated measures the rate at which the water molecules are electrolyzed. Once equilibrium is reached, the rate at which water molecules enter the cell will exactly match the rate at which such molecules are electrolyzed. Consequently, at a given flow rate the water concentration in the gas will be known without any further calibration. An example of a conventional type I electrolytic cell is described in U.S. Pat. No. 4,800,000 to D. Zatko.
A phenomenon called the "recombination effect" can create large errors in the measurement of such electrolytic cells when the sample gas contains substantial amounts of hydrogen or oxygen. The effect refers to the recombination, if a catalyst is present, of hydrogen and oxygen in the cell to re-form water. Thus, a single water molecule can be detected more than once. The catalytic reaction of hydrogen and oxygen will occur on the surface of the precious metal electrodes.
The recombination effect is negligible in inert gases because hydrogen and oxygen produced by electrolysis are present only in very low concentrations. Thus, the probability of molecular collision and reaction of those species is exceedingly small. When the carrier gas contains substantial concentrations of hydrogen or oxygen, however, the probability of reaction increases. In hydrogen gas, for example, oxygen produced by electrolysis can easily collide with surrounding hydrogen and recombine to form water.
The mechanisms for electrolysis and recombination differ. The former is a forced reaction which occurs with energy input at the metal electrodes under the influence of a powerful DC electric field; recombination requires a catalyst and is highly exothermic. Electrolytic hygrometry, the recombination effect, and the factors that affect recombination are described in D. Smith & J. Mitchell, Jr., Aquametry (Part II), pages 661-674, 1056 (2d ed. 1984).
In particular, the Aquametry reference suggests that, because catalysts and catalyst poisons are familiar associates, very small additions of certain catalyst poisons might render the catalytic surfaces more or less completely and permanently inactive. Exposure of the cell electrodes to H.sub.2 S, I.sub.2 vapor, CS.sub.2, HCN, PH.sub.3, and AsH.sub.3 is proposed to potentially eliminate the recombination problem. The reference indicates that I.sub.2 vapor or H.sub.2 S might be good candidates for early trial by injection into the sample gas and HgCl.sub.2 could be added to the cell as a dilute aqueous solution before the H.sub.3 PO.sub.4 solution is admitted to coat the electrode wires.
In addition to the recombination effect discussed above, the response time of an hygrometer is an important issue. Conventional hygrometers tend to react slowly to changes, especially when measuring very small concentrations. One of the reasons for perceived slowness, which has nothing to do with the hygrometer, is the "sticky" nature of the water molecule. This characteristic of water makes small changes slow in reality.
With respect to the electrolytic cell itself, one reason for slow response time is the fact that, in order to reflect a change in the moisture concentration of the sample gas, the moisture content of the phosphoric acid film must change. In an equilibrium situation, a certain state of wetness of the film corresponds to a certain film resistivity and, consequently, to a certain resistance for a given configuration. See D. Smith & J. Mitchell, Jr., supra, at pages 535-36. A new equilibrium state will be approached in an exponential way as the difference between the number of incoming and electrolyzed water molecules becomes increasingly small. It is this difference that makes the film move toward the new state. In general, the less film material, the faster the change because the same number of water molecules represents a larger change in the percentage moisture content of the film.
Non-electrolizable areas of the film will slow the response time considerably. These areas exist where there is no, or an insignificant, electric field such as on top of the electrodes, between the glass and the electrodes, in cavities within the glass, and at the ends of the glass tube. As long as these areas contact electrolizable phosphoric acid, moisture will migrate by diffusion, which is a very slow process.
Another issue is the detection limit. Ultimately, even assuming that the sample gas is perfectly dry and that no other sources of moisture molecules capable of absorption by the phosphoric acid film are present (background), the electrolysis current is still not equal to zero. The film moisture content at any moment has some value corresponding to some resistance and, consequently, produces an electrolysis current. If no moisture enters with the gas, this film moisture content will go toward zero at an increasingly slow rate. When further dry-down is measured in days, the achieved level is considered the "stable" background of the cell. The resistance related to this "background" will be lower when there is less film material and with less phosphoric acid in the non-electrolizable areas such as those mentioned above.
An unacceptable time lag of the hygrometer may occur, especially in response to a rise in moisture concentration, after the hygrometer is connected to a very dry gas for a long period. The presence of dry gas for a long time will cause the components of the hygrometer which contact the gas to become dry themselves. Those components include plastic (e.g., polytetrafluoroethylene (PTFE)) tubes and packing materials, like epoxy, which are known to be relatively porous and to absorb or emit moisture from or into a passing gas stream. Such components are described in U.S. Pat. No. 5,198,094 issued to J. Mettes. In addition, part of the moisture coming into a previously dry cell will be absorbed by non-electrolizable phosphoric acid, such as that present on top of the electrodes, and will impact the electrolysis current only after the moisture diffuses toward electrolizable areas.
When an hygrometer encounters a dry gas having a moisture concentration below its detection limit, the instrument will produce a background level reading. In contrast to that reading and in reality, however, the hygrometer and its components will attain an equilibrium corresponding to the lower (undetectable) moisture level. When the moisture concentration subsequently changes to a higher level, certain internal components of the hygrometer will, because they are dry, absorb the moisture before the gas reaches the analyzer. Consequently, it will be some time before the hygrometer senses the increased moisture and can activate an alarm or show the higher concentration.
The amount of time depends, among other things, on how dry the gas was and on how long the dry gas flowed. The process monitored by the hygrometer may be using gas with an unacceptably high moisture concentration for a relatively long time, therefore, before the hygrometer "reads" the correct concentration and activates an alarm. For many applications, such a time lag is unacceptable.
To overcome the problem of recombination in, and to reduce the response time of, a type I electrolytic hygrometer, a new design for and process of manufacturing the detecting unit of the hygrometer is provided. Accordingly, one object of the present invention is to assure minimal recombination errors when an electrolytic hygrometer is used. Another object is to reduce the response time required for an hygrometer to detect and measure an increase in the moisture concentration of the sample gas measured by the hygrometer. A related object is to provide an hygrometer with a low detection limit. It is still another object of the present invention to render the process of manufacturing the detecting unit of the electrolytic hygrometer easier, faster, more reproducible, of higher yield, and less expensive.